#include <iostream>

using namespace std;

const long long MAX=10001;
const long long INF=0x7FFFFFFF;

int N,S;
int C[MAX],Y[MAX];
long long mincost,percost,totalcost;

void init()
{
	int i;
	freopen("factory.in","r",stdin);
	freopen("factory.out","w",stdout);
	scanf("%d%d",&N,&S);
	for (i=1;i<=N;i++)
	{
		scanf("%d%d",&C[i],&Y[i]);
	}
}

void deal()
{
	int i,j;
	for (i=1;i<=N;i++)
	{
		mincost=INF;
		for (j=1;j<=i;j++)
		{
			percost=(i-j)*S+C[j];
			if (percost<mincost)
				mincost=percost;
		}
		totalcost+=mincost*Y[i];
	}
}

int main()
{
	init();
	deal();
	cout << totalcost << endl;
	return 0;
}